For pregnant women, working the late shift may be less than ideal, researchers report.

In a study from Denmark, pregnant women who worked the night shift were more likely to give birth past their due date. The study also showed that women who did shift work had a slightly increased risk of giving birth to a baby that was smaller than average.

The results of the study appear in the American Journal of Obstetrics and Gynecology.

Research on the effect of shift work or night shifts on pregnancy has been mixed. Some studies have associated shift work with premature births, low birth weight and impaired fetal growth, but other studies have found no such link.

Dr. Jin Liang Zhu and colleagues at the University of Aarhus examined pregnant women’s work schedules in a study that included more than 40,000 Danish women who gave birth between 1998 and 2001. Women were interviewed twice during pregnancy to find out about their work schedules.

The team expected that rotating shift work that included night shifts would have the greatest impact on the risk of premature birth. They also speculated that changing schedules may disrupt the normal rhythm of hormones in pregnant women.

Zhu’s team found that women who worked the night shift on a regular basis had a high risk of carrying their baby past their due date. This increased risk was most pronounced in women with industrial jobs.

The study also found that women whose regular shift was the evening shift were more likely to have a low-birthweight baby.

The biggest impact of evening shifts may be on women’s family and social lives, the researchers suggest. But night work may interfere with the circadian rhythm. This internal clock controls when we sleep and wake and plays a role in other biological processes as well, such as temperature regulation and hormone production.

By throwing the internal clock out of whack, night work may interfere with the production of hormones such as melatonin that play a role in reproduction, according to the report.

Rotating shift workers also had a slightly increased risk of having a low-birthweight child. But unlike most previous studies, the study did not show any association between shift work and the risk of premature and very premature births.

“This discrepancy could be due to differences in working conditions,” the authors suggest. They point out that shift workers in Denmark usually are given extra free time and shorter work hours to make up for their inconvenient schedules.

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SOURCE: American Journal of Obstetrics and Gynecology, July 2004.